ilogb (3)
Leading comments
Copyright 2004 Andries Brouwer <aeb@cwi.nl>. and Copyright 2008, Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived wor...
NAME
ilogb, ilogbf, ilogbl  get integer exponent of a floatingpoint valueSYNOPSIS
#include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
ilogb():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 500 
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED 
_ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
ilogbf(), ilogbl():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
DESCRIPTION
These functions return the exponent part of their argument as a signed integer. When no error occurs, these functions are equivalent to the corresponding logb(3) functions, cast to int.RETURN VALUE
On success, these functions return the exponent of x, as a signed integer.If x is zero, then a domain error occurs, and the functions return FP_ILOGB0.
If x is a NaN, then a domain error occurs, and the functions return FP_ILOGBNAN.
If x is negative infinity or positive infinity, then a domain error occurs, and the functions return INT_MAX.
ERRORS
See math_error(7) for information on how to determine whether an error has occurred when calling these functions.The following errors can occur:
 Domain error: x is 0 or a NaN
 An invalid floatingpoint exception (FE_INVALID) is raised, and errno is set to EDOM (but see BUGS).
 Domain error: x is an infinity
 An invalid floatingpoint exception (FE_INVALID) is raised, and errno is set to EDOM (but see BUGS).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
ilogb(), ilogbf(), ilogbl()  Thread safety  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.BUGS
Before version 2.16, the following bugs existed in the glibc implementation of these functions: *
 The domain error case where x is 0 or a NaN did not cause errno to be set or (on some architectures) raise a floatingpoint exception.
 *
 The domain error case where x is an infinity did not cause errno to be set or raise a floatingpoint exception.